The invention relates to an electrohydraulic motor pump aggregate, to an attachable element and to a pressure limiting valve.
Electrohydraulic motor pump aggregates according to DE 200 07 554 U are employed e.g. in tooling machines for controlling the actuation of hydro-mechanic clamping devices or tool clamping devices or work piece holding devices. Moreover, electrohydraulic motor pump aggregates are used as portable supply units for cutting apparatuses or riveting devices, for tools or for screwing tools. The motor pump aggregate is equipped with an attached element provided with at least one consumer port to which the respective consumer is connected, e.g. via hydraulic tubes or pipings. When the motor pump aggregate is switched off a considerable system pressure will be maintained for longer time due to the high tightness of the valve components and also because of the manufacturing precision connected to hydraulic consumers. Particularly in case of transportable electrohydraulic motor pump aggregates intended to generate maximum pressures up to 800 bars the pressure maintaining property may constitute an undesirable drawback of the high quality of the co-operating hydraulic components, and particularly in case of electrohydraulic motor pump aggregates equipped with an electric AC-motor. Particularly in case of an AC-motor maintained pressure of considerable magnitude complicates the restart after an interruption of the operation. Furthermore, tube couplings or piping couplings hardly can be separated. The hydro-consumer, as long as it is connected to the aggregate, cannot be returned manually even though this might be desirable for a certain reason. A main reason for the long time-pressure maintaining property is that the respective valve components provide leakage free blocking positions. Particularly pressure limiting valves constructed as seated valves are completely free of leakage in the blocking position. On the other hand, leakage free valve components are a must for operating pressures up to 800 bars in order to reach the operating pressure reliably and rapidly with a AC-motor and a small flow rate of the high pressure pump stage.
In hydraulic control systems according to U.S. Pat. No. 4,197,705 A or U.S. Pat. No. 5,454,223 A which are supplied by a regulated variable displacement pump the discharge volume and/or the discharge flow of which is regulated depending on the load, i.e., depending on the pressure demand. A restart of the electromotor after a standstill period only does not cause problems, if all directional control valves are switched into the neutral position, because then a substantially pressureless circulation flow path is open to the tank. The variable displacement pump is adjusted by spring load for maximum discharge volume when restarting. Since the flow resistance of the substantially pressureless circulation flow path is low, no excessive restarting current is needed if the drive motor of the variable displacement pump is an electromotor. However, if one of the directional control valves still is in a control position after a standstill period, then the pressureless circulation flow path is blocked. The variable displacement pump, adjusted by spring load for maximum discharge volume, then has to discharge counter to a considerable flow resistance resulting in an excessive restarting current for the electromotor driving the variable displacement pump.
Further prior art is contained in U.S. Pat. No. 4,362,084 A.
It is an object of the invention to provide a hydraulic motor pump aggregate of the kind as disclosed in which, in a structurally simple way, the long time pressure maintaining property of the hydraulic valve components is eliminated, further to provide an attachable property of the hydraulic valve components is eliminated, further to provide an attachable element for a hydraulic pump aggregate particularly operated by an AC-motor, by which attachable element restart problems of the AC-motors can be avoided, and finally to provide a pressure limiting valve designed as a seated valve which allows an intended leakage in its blocking position.
The pressure relieving device is automatically relieving the pressure as soon as the electromotor has come to a standstill. This considerably facilitates the restart of the electromotor. The effect is of particular advantage in case of an AC-motor, because, otherwise, a restarting AC-motor would consume a relatively high starting current, which occasionally could overload the fuses or the protections provided in the normal power net. Particularly AC-motors are used in case of transportable motor pump aggregates because then the motor pump aggregate can be connected to the normal electricity net during its mobile application. However, the facilitated restart may be desirable for other types of electromotors as well. As further advantages, hose couplings or piping couplings can be separated without problems after relieving the pressure, and the consumer occasionally can be returned manually which might be of advantage e.g. in case of an accidental current breakdown.
The attachable element can be employed to retrofit a electrohydraulic motor pump aggregate in order to facilitate the restart of the electromotor after a break, e.g. in case of an AC-motor. The permanently open by-pass means only negligible loss. No special valve component is employed for this additional function, but the anyway necessary pressure limiting valve.
Despite its seated valve construction the pressure limiting valve generates in its theoretically leakage free blocking position a leakage of a predetermined magnitude which leakage, however, does not affect the pressure limiting function detrimentally. In this way the first maintained pressure will be relieved within a known time duration and without an additional valve component by the pressure limiting valve itself. This offers advantages in case of an electrohydraulic motor pump aggregate containing an AC-motor as a drive source for the pump stage, in order to keep the restarting current of the AC-motor low.
Expediently, the pressure relieving device is incorporated into the attachable element. The permanently open by-pass means a loss during operation which loss, however, is negligible. The pressure is relieved automatically as soon as the electromotor is switched off.
In order to achieve the desired function of the pressure relieving device without an additional valve component, expediently, the by-pass is provided in one of the pressure limiting valves. During operation the pressure limiting valve fulfils the normal pressure limiting function. At the beginning of an operational break the pressure limiting valve additionally is fulfilling the function of the pressure relief.
In case of an electrohydraulic motor pump aggregate having at least two consumer ports and a pressure limiting valve associated to each port both pressure limiting valves conventionally are set for differing response pressures. Expediently, the by-pass is provided in the pressure limiting valve operating with the lower response pressure to avoid losses during the build-up of the higher operation pressure. The loss caused by the by-pass is without importance during a return stroke or release stroke of a hydraulic consumer as controlled by the motor pump aggregate.
An AC-motor used as the electromotor has the advantage to be connectable to the normal AC-electricity net. The dual circuit pump stage allows to first rapidly reach a low pressure level by the lower pressure pump stage and with a relatively high flow rate, and then to reach a very high operational pressure, e.g. up to about 800 bars with the high pressure pump stage but with a small flow rate. The directions of movement of the hydraulic consumer are controlled by means of the multi-ways control valve, while each pressure limiting valve is limiting the maximum operational pressure in one direction of movement. In order to achieve the pressure relieving function the by-pass is provided in one of the pressure limiting valves, expediently in the pressure relieving valve provided for a function of less importance or for limiting a lower pressure.
In a structurally simple way the by-pass is formed in a seated cone valve by a flat surface in the cone jacket of the valve cone. In the blocking position of the pressure limiting valve the flat surface forms a passage or a throttle opening of a predetermined size with the valve seat through which passage or throttle opening the leakage will flow away. Expediently the flat surface is formed by a ground surface which is easy to manufacture.
The flat surface should be located parallel to the generatrice of the cone such that it can be formed precisely during the grinding strokes when grinding the cone jacket, particularly by once interrupting the rotational movement of the valve cone and by an additional positive inward adjustment of the grinding tool.
Alternatively, the by-pass could be formed in seated cone valve or a seated ball valve by at least one notch formed in the valve seat itself.
Expediently the by-pass is dimensioned such that it generates a leakage flow of about 0.3 to 0.4 liter per minute at a reference pressure of 150 bars. In this way a substantial pressure relieve can be carried out within a few seconds, e.g. within 5 seconds.
It is expedient to form the valve cone with a damping piston in order to avoid a nervous response behaviour of the pressure limiting valve.
The damping effect can be increased by an outwardly open cavity of the damping piston, a channel, and integrated a check valve.